1. Field of the Invention
The present invention relates to a back light system and a liquid crystal display device using the back light system.
2. Description of the Related Art
As a liquid crystal display device, a liquid crystal display device placing a back light system as a light source behind a liquid crystal cell is known. As the back light system, a light emitting diode, an organic electroluminescence light emitting device, etc. other than a back light system using a discharge type lamp such as a cold cathode fluorescent tube are known. The large amount of power consumption of the liquid crystal display device is attributed to the power consumption of the back light system. Either in the case where the back light system is composed using a discharge type lamp, a light emitting diode, or an organic electroluminescence light emitting device is the same as this. In particular, in the liquid crystal display device used for a mobile phone terminal, since the capacity of a battery storage for a power supply is very small, the power consumption of a back light system influences the driving period for the liquid crystal display device greatly.
As a measure for reducing the power consumption for the back light system, using a highly efficient light emitting diode, or devising the optical system of a light guide plate etc. and extracting light efficiently with little electric power is achieved. As other measures, the technology, which modulates the light of the back light system according to the surrounding luminosity, and prevents use of maximum luminance in a dark environment is known (for example, refer to Patent Document 1).
However, even when using the technology mentioned above, since the power consumption of the back light system is large, a further reduction of the power consumption for the back light system is desired. Moreover, when the luminance of the back light system is merely reduced low, there is a problem that contrast becomes small in the liquid crystal display, and then display quality deteriorates.
Patent Document 1: Japanese Patent Application Laying-Open Publication No. H10-282470